You can't. We have no control over which images Facebook chooses to display and we have no control over how long it takes for the image caches at Facebook to be flushed.
They make changes frequently and what follows may or may not be where they are at today:
Only the first five images uploaded to a post (direct inclusion/gallery/slideshow) will be sent to Facebook. The images size has to be at least 200 x 200 pixels.
To select which image will be used for things like Facebook Publicize, you can use a theme that supports featured images. http://en.support.wordpress.com/featured-images/ If a featured image isn’t set or your theme doesn’t use featured images, then the code tries to pick the first image in the post or page. If no image is found, then the blavatar is used. http://en.support.wordpress.com/avatars/blavatars/
See also > http://en.forums.wordpress.com/topic/fb-link-has-no-thumbnails?replies=4#post-93686